can somebody explain how the answer of this question is B

(A) This answer choice describes a suggestion that some people give to older people. The suggestion is based on a particular assumption: that older people need mental exercise in order to stay sharp. However, even though people buy into this assumption and even give advice based on it, it could still be incorrect. The well-meaning advisors could actually be basing their advice on incorrect information.

(B) CORRECT. The argument claims that social contact causes better mental skills. This might not actually be the case. If illness limits both social contact and mental acuity, then the study would be correct in noting that social contact is associated with better mental skills. However, it would be wrong to conclude that one causes the other, since a third factor—good health—actually causes both of them (i.e. healthy people are more likely to have strong mental sharpness and significant social interactions).

(C) This doesn’t weaken the link between social contact and mental sharpness. The answer choice doesn’t specify whether these proficient people are or aren’t mentally sharp. If they do maintain their mental sharpness, it could still be attributable to social contact, mental exercise, both, or neither. (Also, note that someone who is proficient in solving mathematical problems doesn’t necessarily solve math problems regularly.)

(D) This answer choice attacks the methods used by a study, rather than that study’s conclusions. Typically, as in this case, these answer choices are incorrect. The issue is that, only using the information in the argument and the answer choice, there’s no way to know whether studies that analyze data are less accurate than studies that collect original data. You may have personal beliefs about the matter, but the right answer to a Critical Reasoning problem will only ever use information found within the argument, with very few exceptions.

(E) The conclusion states that social contact improves mental skills. This could be true regardless of how mental skills are tested. Perhaps social contact improves performance on math problems.


Additional Analysis About Option B)
The key to understanding why option B weakens the argument requires paraphrasing the conclusion, and taking that paraphrase an extra step (and that’s the core reason why this question is up there on the difficulty spectrum):

The argument is essentially saying that social interaction boosts mental sharpness.

Paraphrased conclusion
That claim could be paraphrased to say: without social interaction, mental sharpness would be reduced.

Now read B):
Many medical conditions and treatments that adversely affect a person's mental sharpness also tend to increase that person's social isolation.

And since B) has a classic GMAT style double-negative, let's paraphrase B) too:
Many medical conditions and treatments that reduce mental sharpness also tend to reduce social interaction.

That means B) is saying:
Medical Conditions and Treatments ➧ Mental Sharpness ⬇, Social Interaction ⬇.

So option B is introducing this Medical Conditions and Treatments alternative cause that impacts both social interaction and mental sharpness. So it's not that social ➧ mental, they go hand in hand attributable to another cause, thereby weakening the argument.